EP0798078A2

EP0798078A2 - Method for manufacturing backing pad and apparatus used therefor

Info

Publication number: EP0798078A2
Application number: EP97302066A
Authority: EP
Inventors: Hiromasa Hashimoto; Koichi Saito; Koji Morita
Original assignee: Shin Etsu Handotai Co Ltd
Current assignee: Shin Etsu Handotai Co Ltd
Priority date: 1996-03-28
Filing date: 1997-03-26
Publication date: 1997-10-01
Also published as: EP0798078A3; TW348280B; JPH09262763A

Abstract

A method and an apparatus for manufacturing a grooved backing pad suitable for use in polishing of semiconductor wafer based on the waxless method. The backing pad manufacturing apparatus contains a load/unload conveyer 11 and a groove forming unit 21 above a chuck table 4. The main body 22 of the groove forming unit comprises an aluminium-made hollow axis 26 and a plurality of aluminium-made round flanges arrayed thereon. These round flanges having an almost equal diameter are aligned concentrically with the hollow axis and fixed thereon along its longitudinal direction. In a fabrication process of the grooved backing pad, a work fixed on a sheeted elastic member is vacuum-chucked on the chuck table using the load/unload conveyer, steam is supplied through the hollow axis 26 to heat the round flanges 27 to the softening point of the sheeted elastic member or above, and the round flanges are roll-contacted to the surface of the sheeted elastic member under pressure by traveling the main body 22 of the groove forming unit along the surface of the member.

Description

The present disclosure relates to subject matter contained in Japanese patent application No. 104371 (filed on March 28, 1996) which is expressly incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing backing pad suitable for polishing semiconductor wafers by the waxless method and a manufacturing apparatus, in particular to a method and an apparatus for carving a number of grooves on a wafer holding surface of a sheeted elastic member.

2. Description of the Related Art

Semiconductor wafers are mirror-polished generally in their finishing process by so-called chemical mechanical polishing process. Most popular methods for supporting the wafers onto polishing carriers in this mirror polishing include the wax process and the waxless process, in which the former process involves the use of adhesion wax spread on one surface thereof, and the latter process involves either the use of vacuum chuck or water-aided holdihng on a backing pad made of porous resin such as polyurethane foam material.
As shown in Fig.5, a polishing apparatus 51 used in the waxless process based on water-aided holding comprises a polishing load 52, a template 53, a polishing pad 54 and a turn table 55. The template 53 further consists of a polishing plate 56, backing pad 57 and a template blank 58, all of which layered in this order.
The polishing plate 56 is formed as a round ceramic plate. The backing pad 57 is made of polyurethane foam material having a flat and smooth bottom surface (facing to the table side) provided as a wafer holding surface 37a and a porous content with a great number of isolated pores. The template blank 58 is referred as a laminated glass fiber plate immersed, for example, with epoxy resin, and has engagement holes 58a bored therein for holding a wafer. The backing pad 57 is fixed on the polishing plate 56, the template blank 58 fixed on the backing pad 57, and the polishing pad 34 fixed on the turn table 55, respectively, using adhesive.
Before starting wafer polishing with this polishing apparatus, a semiconductor wafer W is fixed on the template 53 according to the procedures given below. First the wafer holding surface 57a the backing pad 57 is applied with water at around the engagement holes 58a of the template blank 58, removed with excessive water, and toward which the back surface of a wafer is then pressed while supporting the wafer center so that the air is prevented from intruding between the wafer holding surface and the back surface (opposite to the surface to be polished) of the wafer. The semiconductor wafer W is, as shown in Fig.5, adsorbed and fixed on the wafer holding surface 57a of the backing pad 57 assisted by surface tension of the water.
It is thus necessary for fixing the wafer to retain an appropriate amount of water in a gap formed between the wafer holding surface 57a and the back surface of the wafer. The conventional backing pad 57, however, could not prevent some air or excessive water from intruding and being retained in the gap at the time of wafer loading. Retained air and water are indicated by symbol 61 in Fig.6.
In recent years, diameter of semiconductor wafer is becoming larger and required flatness after polishing is thus becoming more stringent as compared with that for small-sized wafers. In larger diameter wafers of 8 to 12 inch diameter, however, influences stronger than those on smaller wafers would be brought on the flatness of polished wafer by air or excessive water, not required for adsorbing the wafer, retained in the gap between the wafer holding surface of the backing pad and the back surface of the wafer. Larger diameter wafers also suffered from difficulties in thorough removal of the above air and the excessive water, thus making it difficult to produce large-sized polished wafers with an excellent flatness.
That is, when a backing pad of the conventional structure is used to polish a large-sized wafer, the above gap might trap the air and 61 as shown in Fig.6, thus allowing the wafer W to be polished as convexed toward the polishing pad 54, which would result in nonuniform thickness in the polishing stock removal 62. Thus the polished surface Wa of the wafer W would be concaved in its center since the center portion is polished excessively as compared with the peripheral portion, which has been pointed out as a demerit.
One solution of this problem employed a polyurethane foam material-made backing pad as a wafer holding member to avoid trapping of the air or water into the gap. Most of the inner isolated pores were, however, formed along the direction of the thickness of the backing pad and allowed only insufficient communications among the isolated pores, thus the air removal has been unsatisfactory.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a backing pad which can ensure an excellent flatness of the mirror finished surface of wafers even if large-sized wafers are used.
It is another object of the present invention to provide a manufacturing of a backing pad having a number of grooves on a wafer holding surface of a sheeted elastic member, and an apparatus suitable for carrying out the above method.
In accordance with an aspect of the present invention, the above object is attained by providing a method for manufacturing a backing pad used for fixing semiconductor wafer to be polished based on the waxless method and having a number of grooves on the wafer holding surface of the sheeted elastic member, in which the grooves are formed by using a groove forming member having a plurality of round flanges with almost equal diameters (or the same diameter) arrayed at proper intervals along and concentrically with an axial body, heating the round flanges of the groove forming member to a temperature higher than the softening point of the sheeted elastic member , traveling the member over the surface of the sheeted elastic member to make the flanges roll under pressure on the sheeted elastic member, which results in simultaneous formation of a plurality of grooves having a shape and a size corresponded to the outer edge of the round flanges.
Thus obtained backing pad enables automatic and quick discharge, upon wafer loading, of the air and the excessive water once trapped in a gap between the wafer holding surface of the backing pad and the wafer back when a semiconductor wafer is fixed on the backing pad with aid of surface tension of water.
In accordance with another aspect of the present invention, the above object is attained by providing a backing pad manufacturing apparatus comprising a fixing member made of sheeted elastic material, and a groove forming unit having a plurality of metallic round flanges with an almost equal diameters arrayed at proper intervals along and concentrically with an axial body, the axial body being rotatable as well as reciprocative in the direction parallel to a sheeted elastic material fixing plane of the fixing member, and the plural number of round flanges are heatable to a temperature higher than the softening point of the sheeted elastic member.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become apparent from the following detailed description when taken with the accompanying drawings in which:

Fig.1 shows a front view for explaining a backing pad manufacturing apparatus in accordance with the present invention.
Fig.2 shows a right side view of Fig.1
Fig.3 shows a plan view of a morphology of a backing pad manufactured with the apparatus shown in Fig.1.
Fig.4 shows a graphical sectional view for explaining action of semiconductor wafer polishing using the backing pad as shown in Fig.3 based on the waxless method.
Fig.5 shows a sectional view for explaining a semiconductor wafer polishing apparatus using the conventional backing pad based on the waxless method.
Fig.6 shows a graphical sectional view for explaining action of semiconductor wafer polishing using the backing pad as shown in Fig.5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be detailed in connection with a preferred embodiment.

Example 1

Fig.1 shows a front view for explaining a backing pad manufacturing apparatus of the present invention, and Fig.2 is a right side view of Fig.1. As illustrated in these figures, on the lower part of a base frame 1, is installed a vacuum chuck table 4 rotatable at a 90° pitch with aid of a drive unit 3 equipped with a motor 2. A top surface of the chuck table 4 is perforated with a number of vacuum suction holes (not shown), the holes being communicated to the suction side of a vacuum pump (not shown). The top surface of the chuck table 4 is aligned in the horizontal direction. On the upper part of the base frame 1, are equipped a load/unload conveyer 11 and a groove forming unit 21 for carving a number of straight grooves on the wafer holding surface of the sheeted elastic member. A timing belt (not shown) is wound around a grooved pulley 5 driven by the motor 2 and a grooved pulley 7 connected to a rotation axis.
The load/unload conveyer 11 comprises a metal plate 14 reciprocated (in the lateral direction in Fig.1) by an air cylinder 12 and a metal plate 15 reciprocated by an air cylinder 13, both of which aligned in a rightly opposing manner in a horizontal plane to enable adjustment of gap between these metal plates 14 and 15. The front edge faces (opposing faces) of the metal plates 14 and 15 are concaved in an arc form in a flat view, so that a nearly round through hole appears when the gap is minimized.
The groove forming unit 21 comprises a frame 24 having wheels 24a in its bottom part, a main body 22 mounted on the lower part of the frame 24, and a friction roller 23 made of heat-resistant rubber and mounted on the upper part of the frame 24, so that the frame 24 is reciprocated (in the lateral direction in Fig.2) by a drive unit (not shown) along a straight rail 25 extended in a horizontal direction. The main body 22 comprises an aluminum-made hollow axis 26 supported by the frame 24 in a freely rotatable manner, and a plurality of aluminum-made round flanges 27 immobilized on the hollow axis 26. These round flanges 27 having an equal diameter are aligned concentrically and orthogonally to the hollow axis 26, with the intervals between adjacent outer edges of the round flanges 27 kept constant. The friction roller 23 is supported on the frame 24 in a freely rotatable manner, and can contact its peripheral face with all of the round flanges 27 with pressure resulted from its own weight, by which residue of the sheeted elastic member adhered on the circumference of the round flanges 27 can be removed. The gap between the outer edge face of the round flanges 27 and top surface (vacuum chuck plane) of the chuck table 4 need be fine-adjustable, thus the design involves fine adjustment of a fixation height of the rotation axis of the hollow axis 26.
The hollow axis 26 is designed to accomodate a heat medium, for example, steam flow at a proper temperature, or to have a heater. Thus, upon reciprocating motion of the main body 22, the round flanges 27 are brought into rolling contact with the sheeted elastic member and allowed to rotate like an idle wheel.
The paragraphs below describe procedures for fabricating a backing pad carved with a number of grooves on the wafer holding surface of the sheeted elastic member, and the action thereof.

(1) Fixing a sheeted elastic member on the chuck table.
The groove forming unit 21 is held in a standby position on the edge of the rail 25 as shown in Fig.2. A ceramic plate 28 is obtained and laminated on its upper surface with a round sheeted elastic member with an equal diameter using an adhesive or double-side-coated adhesive tape, to provide a work 29. The wafer holding surface is previously flattened and smoothened by, for example, precision surface grinding.
The air cylinders 12 and 13 are then operated to minimize the gap between the metal plates 14 and 15, by which a nearly round through hole appears between the plates. The work 29 is placed on the front edges of the metal plates 14 and 15, and then loaded on the top surface (vacuum chuck plane) of the chuck table 4 after carried by the load/unload conveyer 11. Figs. 1 and 2 show a state of the loading.
The work 29 is elevated by an air cylinder 6a from the plates 14 and 15, the plates 14 and 15 driven by the air cylinders 12 and 13 were retracted from the work 29, and the air cylinder 6a is descended to place the work 29 on the chuck table 4. The work 29 is then vacuum chucked on the chuck table 4 using the vacuum pump. The gap between the outer periphery (lower edge) of the round flanges 27 and the top surface of the chuck table is finely adjusted by previously adjusting the fixation height of the rotation axis of the hollow axis 26. Thus the depth of grooves to be carved on the wafer holding surface of the sheeted elastic member can be set at a proper value.
(2) Carving grooves on the wafer holding surface of the sheeted elastic member

The hollow axis 26 of the main body 22 was supplied with steam to raise its temperature equals to the softening point of the sheeted elastic member or above (melting point, for example). The friction roller 23 was then placed by its own weight on the outer periphery of the round flanges 27, and the frame 24, or substantially the groove forming unit 21, is traveled on the straight rail 25 from the starting position to the ending position.
The round flanges 27 are thus rolled while being kept in pressure-contact on the wafer holding surface of the sheeted elastic member. Since the wafer holding surface brought into contact with the outer periphery of the round flanges 27 is locally heated to be softened as well as pressed, a plurality of straight grooves having a width almost equals to that for the outer periphery of the round flange 27 are formed at a time, while generating an intergroove pitch almost same as the interval between adjacent outer edges of the round flanges 27. Fig.2 shows a status in which the groove forming unit 21 is rested on the starting position of the rail 25.
The chuck table 4 is then rotated at 90° , while the rotation of the chuck table 4 being not interfered since the groove forming unit 21 being rested on the end position of the rail 25, and the forming unit 21 is traveled again from the end position to the starting position.
According to the above procedures, a backing pad is successfully obtained, in which the wafer holding surface 30a of the sheeted member 30 is smoothly finished and thereon a number of grooves 32 are carved in a lattice form,
All the grooves 32 run straight toward the outer periphery of the sheeted elastic member 30 and have an equal width and an equal depth, both values being kept in constant along the longitudinal direction of the grooves 32. An intergroove pitch is also kept in constant over the entire of the wafer holding surface 30a. The backing pad 31 is, as indicated for example by two-dot chain line in Fig.3, formed in a diameter as large enough to afford a plurality of wafers W, W .... (five slices in Fig.3 for example). As a material for forming the sheeted elastic member 30, known materials including the above-mentioned polyurethane foam material are available (see the descriptions below).
A most preferable layout of the groove 32 is referred as a square lattice, but an oblique lattice and parallel linear (stripe) pattern are also allowable by request. In the case with the stripe pattern, it is not necessary to rotate the chuck table 4. To form the oblique lattice grooves using the apparatus described in the above embodiment, first the groove forming unit 21 is traveled along the rail 25 from its starting position to the end, the chuck table is then rotated at an arbitrary angle, and again the groove forming unit 21 is traveled along the rail 25 from its starting position to the end position.
The backing pad 31 does not absolutely require that all the grooves 32 run throughout the edge of the sheeted elastic member 30 as described above, but only require that all or nearly all of the grooves 32 reach at least the edge of the wafers.
This is because a most principal matter relates to smooth discharge of the air and water trapped in a gap between the wafers and the wafer holding surface 1a of the backing pad 1 when the wafers are loaded. The grooves 32 are thus not necessarily be formed in geometrically straight but waviness to some degree is also permissible. The thickness of the backing pad 31, as well as the depth, width and intergroove pitch of the grooves 32 can properly be set considering mainly diameter of semiconductor wafer.
A typical fabrication process of the sheeted elastic member 30 is as the following.
A commercial uniform-thick elastic sheet made of polyurethane foam material having internally a great number of isolated pores is employed, one surface of which is designated as a wafer holding surface (more correctly a surface functions as a wafer holding surface after fabricated into a backing pad), the wafer holding surface is ground into flat with a surface grinder, and the sheet is cut into a round plate with a predetermined diameter. The round plate is then fixed on a ceramics-made polishing plate with an adhesive, and smoothened on its wafer holding surface using a precision surface grinder (product of Shibayama Kikai Co., Ltd.).
According to the semiconductor wafer polishing using the above backing pad 31 based on the waxless method, in which wafer is supported on the backing pad 31 with aid of surface tension of water, the air and the excessive water once trapped into a gap between the wafer holding surface 1a of the backing pad 31 and the back surface of each wafer W are automatically routed to the groove 2 and discharged from the edge of the wafers W. This allows the polished surface Wa of the wafer W kept during the process in parallel with the polishing surface of the polishing pad 34, which will, without difficulty, result in an uniform thickness of a polishing stock removal 33 and an advanced flatness in the polished surface of the wafer W even when large-sized wafers are used.
In a manufacturing process of a backing pad of the present invention, as shown in Fig.3 or as described later in [Test Example 1], diameter or other specifications of a single backing pad is so set to afford a plurality of wafers at the same time. In another embodiment, a template blank bored with a plurality of round through holes are fixed on the polishing plate and the backing pad is fixed thereon by insertion into these through holes, so that each backing pad can hold a single semiconductor wafer.
The following paragraphs describe an example of test polishing in which silicon wafers of 8 inch diameter and 0.735 mm thick were polished using the backing pad fabricated using the above-mentioned apparatus.

[Test Example 1]

The round backing pad made of polyurethane foam material as fabricated in the first embodiment and as shown in Fig.3 was used. It was 565 mm diameter, 0.3 mm thick and having a pore size of 10 to 30 µm. The groove 32 aligned on the wafer holding surface of the backing pad in a lattice pattern (all openings of the lattice are square-shaped) had a width of 0.5 mm, an intergroove pitch of 15 mm and a depth of 0.3 mm, each value being kept constant over the entire of the wafer holding surface, and its width and depth also kept unchanged within the longitudinal direction.
As shown in Figs.3 and 4, the round plate backing pad 31 was fixed on a ceramic polishing plate 56 having a nearly equal diameter using an adhesive, on which the glass fiber reinforced epoxy resin-made template blank 58 having five engagement holes aligned on a single circumference for fixing wafers was fixed also with the adhesive. A surface area within which the round through holes are exposed can thus serve as a wafer holding surface.
The wafer holding surface 30a of the backing pad 31 was then applied with water, removed therefrom with an excessive water, and pressed thereto with the back surface of a wafer in such a manner that the wafer center being supported. The silicon wafer W was adherently fixed onto the backing pad 31 with aid of surface tension of water, while some air and excessive water trapped between the wafer holding surface 30a and the back surface of the wafer were automatically and quickly discharged out of the backing pad 31 through the grooves 32. The polished surface Wa was thus maintained in parallel with the polishing surface of the polishing pad 54 as shown in Fig.4.
After these preparatory steps before wafer polishing, the polishing load 52 was descended so that the polished surface Wa of the silicon wafer W is brought into contact and pressed with the polishing pad 54 at an predetermined pressure. Under a supply of polishing slurry (not shown) containing fine abrasive grain, the polishing load 52 was allowed to rotate as well as revolve, and the turn table 55 was rotated in the counter direction as that for the rotation of the polishing load 52, to effect a simultaneous mirror polishing of five slices of silicon wafers.
Flatness of the polished surfaces of the wafers were evaluated in terms of LTVmax (in µm). LTVmax is defined as a maximum difference between a maximum value and a minimum value of thickness observed in a 20 mm × 20 mm cell divided from a wafer, that is a maximum value of LTV (Local Thickness Variation). Results of the measurement using a general thickness gauge were shown in Table 1. Table 1

Silicon Wafer

1 2 3 4 5

LTVmax 0.24 0.37 0.35 0.28 0.31

Average of LTVmax 0.31

Comparative Example

Five silicon wafers were simultaneously mirror-polished according to the same procedures as in Example 1 except that using a backing pad having no groove. Results were shown in Table 2. Table 1

Silicon Wafer

1 2 3 4 5

LTVmax 0.36 0.71 0.53 0.81 0.49

Average of LTVmax 0.58
From comparison between Tables 1 and 2, it was found that Comparative Example 1 has a larger variation in LTVmax among the wafers as well as a larger average value of LTVmax. In Example 1 on the contrary, LTVmax was less variable and gave a smaller average value. It was also confirmed that the entire surface of the wafers were polished with an advanced flatness in Example 1, whereas the wafers were concaved in their centers in Comparative Example 1.
It was thus made clear that batch polishing (simultaneous multi-wafer polishing) using a backing pad obtained by a manufacturing method and using a manufacturing apparatus of the present invention can provide mirror-polished wafers with an advanced flatness and a good uniformity in the flatness among the wafers without difficulties.
As will be clear from the foregoing explanation, a manufacturing method and manufacturing apparatus of the present invention employ a groove forming member having a plurality of round flanges arrayed in parallel along an axial body, and make the flanges roll under pressure on the wafer holding surface, while the flanges being heated to a temperature higher than a softening point of the plastic-made sheeted elastic member. Thus a backing pad having a number of grooves on the wafer holding surface of the sheeted elastic member is obtained.
When wafers are fixed on this backing pad with aid of surface tension of water, the air and excessive water once trapped into gaps between the wafer holding surface of the backing pad and wafer backs are routed to the grooves to be discharged from behind the wafers in an automatic and quick manner. Since the polished surface of the wafers are thus supported in parallel to the polishing surface of the polishing pad during the polishing process and polishing allowance is made into an uniform thickness, it is facilitated to obtain an advanced flatness of the mirror-polished surface of the wafers.
While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by the present invention is not limited to those specific embodiments. On the contrary, it is intended to include all alternatives, modifications, and equivalents as can be included within the spirit and scope of the following claims.

Claims

A method for manufacturing a backing pad used for polishing semiconductor wafers based on the waxless process and having a number of grooves carved on a wafer holding surface of a sheeted elastic member by using a groove forming unit, said groove forming unit comprising an axial body and a plurality of round flanges having almost an equal diameter aligned concentrically with and along the longitudinal direction of said axial body at proper intervals, in which
said round flanges are contacted under pressure with said sheeted elastic member and are rotated by driving said groove forming unit along the surface of said member, with said round flanges heated to a temperature as high as or above the softening point of said member, to produce at a same time a plurality of grooves having a shape and dimensions corresponded to those of the outer edges of said round flanges.
A method for manufacturing a backing pad as set forth in claim 1, wherein said grooves are formed in a lattice.
A method for manufacturing a backing pad as set forth in claims 1 or 2, wherein said sheeted elastic member consists of polyurethane foam material including internally a number of isolated pores.
An apparatus for manufacturing a backing pad used for polishing semiconductor wafers based on the waxless process and having a number of grooves carved on a wafer holding surface of a sheeted elastic member, comprising
a fixing member of said sheeted elastic member, and

an axial body along the longitudinal direction of which a plurality of metallic round flanges having almost an equal diameter are concentrically aligned at proper intervals,

said axial body being rotatable,

a plurality of said round flanges being reciprocative in a direction parallel to a sheeted elastic member fixing plane of said fixing member, and

a plurality of said round flanges being heatable to a temperature equal to or above the softening point of said sheeted elastic member.
An apparatus for manufacturing a backing pad as set forth in claim 4, wherein said groove forming unit is so designed to relatively approach or set back from said sheeted elastic member fixing plane of said fixing member, and to enable fine adjustment of the distance.
An apparatus for manufacturing a backing pad as set forth in claims 4 or 5, wherein a linear rail is placed in parallel to said sheeted elastic member fixing plane of said fixing member, on which a frame is mounted in a reciprocative manner, said groove forming unit being mounted on said frame, and said axial body being mounted in parallel with said sheeted elastic member fixing plane of said fixing member and orthogonally to the longitudinal direction of said rail.
An apparatus for manufacturing a backing pad as set forth in claims 4, 5 or 6, wherein a plurality of said round flanges are made of aluminium, and said axial body being made into a hollow axis to allow heat medium at a proper temperature flow therethrough.
An apparatus for manufacturing a backing pad as set forth in claims 4, 5, 6 or 7, wherein said fixing member is a vacuum chuck table being rotatable at a proper pitch angle with aid of a drive unit, and a vacuum chuck plane of said table being set in the horizontal direction.